In a car driven by a gasoline engine or other internal combustion engine requiring an ignition plug, pulsating ignition noises are produced, and a car radio, etc. on board catches such ignition noises and reproduces a sound difficult to hear. In this connection, most car radios have a filter inserted in a power source circuit and include a noise removal device.
FIG. 10 is a block diagram of a prior art pulsating noise removal device.
In a normal conduction without noises, a gate circuit 1 outputs an entered audio signal Vi in the original form as an output signal Vo. A circuit for controlling the gate circuit 1 consists of a high-pass filter (HPF) 11, amplifier (AMP) 12, AGC circuit 13 and monostable multivibrator circuit 14. The HPF 11 removes an audio signal S from the entered signal Vi (audio signal S+pulsating noise N) and permits a pulsating noise N alone to pass through. An output signal of the HPF 11 is amplified by the AMP 12 and subsequently applied to the AGC circuit 13 and the monostable multivibrator circuit 14. The AGC circuit 3 is used to prevent that the gate circuit 1 is continuously blocked and that the output signal Vo is continuously withheld when noise components are continuously detected.
The monostable multivibrator circuit 14 is configured to generate a predetermined constant switching pulse when a pulsating noise is detected, so that the pulse blocks the gate circuit 1 in order to not output the entered signal Vi so as to remove the pulsating noise N. As the gate circuit 1, a pre-holding type or a linear or other signal compensation type is used in most cases.
FIG. 7 shows a waveform of a signal from which a noise has been removed by a pre-holding type gate circuit. Tsw indicates a switching time in which the gate 1 is turned off in order to remove a pulsating noise, and it has a length corresponding to the pulsating noise. T designates a repeat cycle of the puslating noise which actually exhibits various values depending on noise sources.
FIG. 8 shows the signal distortion ratio characteristic against an audio signal frequency upon a noise removal switching of an audio signal. As the signal frequency fs increases, the distortion ratio Ds increases. Also when a linear compensation type is used as the gate circuit 1, the same tendency is seen.
Systems of this type are disclosed in Japanese Patent Publications No. 55-46087 and No. 56-45535.
In the prior art pulsating noise removal devices, however, when the audio signal cycle decreases with respect to the switching time, the gate switching causes an increase in the distortion and causes an inversion phenomenum which gives a user an impression that the noise rather increases.
The inversion phenomenum occurs because the signal takes the form of P1-P2-P3 in the switching time Tsw in which the gate 1 is turned off and loses a portion corresponding to an area As with respect to the original signal. When no signal is removed, a pulsating noise An having an area hatched in the drawing is added to the signal S. The lost signal portion As and the pulsating noise component An both invite a distortion of the original signal.
If the pulsating noise component An is sufficiently large as compared to the lost signal portion As (when a pulsating noise is large), the signal distortion can be decreased by effecting a switching to turn off the gate 1 to remove the noise. Most pulsating noise removal devices aim this effect.
In contrast, if the pulsating noise component An is small with respect to the lost signal portion As (when a pulsating noise is small), a switching action to turn off the gate 1 rather increases the distortion. That is, an inversion phenomenum occurs. Therefore, the noise removal device must be held inoperative in this case. However, the lost area As of the signal caused by a switching varies with the signal frequency. That is, it decreases or increases when the signal frequency is low or high. Therefore, assuming that the pulsating noise magnitude is constant, the noise removal device must effect its removal operation at a low signal frequency but stop its operation at a high frequency causing an inversion phenomenum. However, prior art noise removal devices are not configured in this fashion, and are subject to an inversion phenomenum.